1. Field of the Invention
This invention relates to sound motion picture cameras which utilize photographic film with a magnetic track in recording sound accompaniments for motion pictures, and, more particularly, to an automatic gain control circuit in a sound recording circuit adapted for use with such camera.
2. Description of the Prior Art
Recently, a new type of sound motion picture camera (particularly in the field the 8mm camera art) has appeared on the market which is characterized by use of a composite recording medium consisting of a photographic roll film on which a track of magnetic material is applied for sound recording. This type camera makes use of a recording circuit of construction well known in the tape recorder art. In more detail, such recording circuit is constructed to include an amplifier circuit responsive to a sound signal from a microphone for producing an output signal with a bias signal is applied to a magnetic head with action on the magnetic track. In order to maintain the responsibility of the amplifier circuit at a proper level, it is required to achieve satisfactory gain control either manually or automatically. Because of extreme difficulty of manual gain control in sound motion picture cameras particularly with respect to 8mm cameras, the use of automatic gain control circuit is widely accepted.
Typical of such conventional automatic gain control circuits in the field of the tape recorder art is that shown in FIG. 1, wherein a sound signal from a microphone appears at an input terminal 1, and, after having been attenuated in a ratio of a fixed impedance which may be in the form of a resistor 2 to a variable impedance which may be in the form of a transistor 3, is amplified by an amplifier 4 having an output terminal 4. An automatic gain control circuit for such amplifier circuit is shown as comprising a resistor 6, a capacitor 8 connected through a diode 7 in series to the resistor 6 and to the output terminal 5 of the amplifier 4, and a resistor 9 connected in series to the capacitor 8 at one end thereof, the opposite end of which is connected to the base electrode of the transistor 3, so that the impedance of the transistor 3 is varied depending upon the output of the amplifier 4. When the level of input signal at the terminal 1 becomes higher, therefore, the impedance of the variable impedance element 3 is made the smaller, while when becomes lower, it is made the larger, the resulting output level of the amplifier 4 being maintained constant.
With this automatic gain control circuit, the response time from the moment when a large input signal starts to appear at the input terminal 1 to the moment when a gain control procedure is terminated at a value smaller than the initial one, termed "attack time," is controlled by the resistor 6 and the capacitor 8. On the other hand, the recovery time from the moment when the large input signal disappears to the moment when the gain takes the initial value as it increases is controlled by the capacitor 8, the resistor 9 and the base impedance of the transistor 3. In general, the conventional tape recorders are designed to operate with attack and recovery times specified by taking into account the following facts. If the attack time is too long, sound recording tends to be distorted at the start so that it is preferred that the attack time be reduced to as short a value as possible, but, if the reduction is too much, an instantaneous sound such as a click will cause accidental actuation of the gain control circuit, thus the commonly accepted value of the attack time is on the order of 0.1 second. On the other hand, the recovery time is associated with more difficult aspects in determining a single value than those of the attack time, since the required value of the recovery time depends upon what kind of sound of principle interest is to be recorded. For example, that type of tape recorder which is adapted to record music sound must be operated over a wide dynamic range, or otherwise music will be recorded with a considerable loss in rhythm. In addition thereto, as the music recordings generally continue for a long period of time, the recovery time is required to be as long as possible, usually on the order of 2 to 3 minutes. In contrast to the music recorder, the conversation recorder, because of its aiming at providing the clarity of voices rather than the assurance of a wide dynamic range, is designed to have a comparatively short recovery time of about 2 to 3 seconds, thereby it being made possible to record loud and low voices at almost the same level.
As far as almost all of the tape recorders are concerned, there is no need to employ automatic control of the attack and recovery times in performing sound recordings. This is because the music recorder when rendered cooperative with a sound source having a previously narrowed dynamic range such as a radio, tuner, television set and sound reproducing apparatus without recourse to a microphone will take sound recordings of sufficiently good quality, provided that the selected value of the recovery time is adequately long. When a sound such as music in live performance is desired to be recorded directly from the microphone, in other words, when a sound having a wide dynamic range is to be recorded over that wide dynamic range, use will be made not of the automatic gain control circuit, but of a VU meter by which manual adjustment of sound level is performed to obtain high quality sound recordings. This is because the operator is permitted to focus his attention on the sound recording control and because such manual control leads to a higher possibility of achieving satisfactory results. Where there is no need to secure a wide dynamic range as in conversation recordings, or when loud and low voices are desired to be recorded at almost the same level, a satisfactory result can be secured by adjusting the recovery time to a small value. Besides these types of tape recorders, it is known to provide an alternate type of tape recorder having manually selective control of recovery time.
In recording sound accompaniments for motion pictures, as the photographer is obliged to focus a large proportion of his attention on the motion picture taking, it is difficult to perform manual control of sound level, and therefore, it is preferred to make use of an automatic gain control circuit. When the live sound recording passes through a microphone for the motion picture, and since the music and conversation accompanied with the motion picture and other various sounds present have a wide dynamic range, it becomes very difficult to select a proper value of the recovery time for employment in the automatic gain control circuit. The attack time, however may be adjusted to 0.1 second or less by taking into account the occurrence of distortion in the sound recording at the start in a fashion similar to that of the tape recorder.
In an 8mm camera, for example, each magazine containing a roll film of 50 feet long can take a continuous series of motion picture frames at a film speed of 18 frames/sec. for about 3 minutes and 16 seconds. As the film is intermittently advanced at the picture taking station, production of noise takes place, causing difficulty in incorporating the microphone within the camera housing, so that the microphone must be separate from the camera. (In the case of the tape recorder, because of a low level of the noise resulting from the tape driving, the microphone is not necessarily separate from the tape recorder.) For these reasons, if the recovery time is as long as 2 to 3 minutes, a loud sound followed by low sound can be recorded in a corresponding dynamic range for providing an impression of far and near distance, as the long recovery time assures the maintenance of the gain unchanged at a small value. Even when the sound recordings are performed in a noisy environment, it is possible to reduce the noise relative to the sound of principal interest provided that this sound is sufficiently large. However, when a crashing sound is given to the microphone as by tapping the microphone on a solid body, the gain is later maintained at a reduced value for 2 to 3 minutes so that it is impossible to record the sound of principal interest at a proper level. In a case where an unnecessary large sound occurs at the start of sound-accompanied motion picture taking sequence, all the recording with one magazine cannot be made under proper conditions. On the other hand, if the recovery time is about 2 to 3 seconds, the reduction of the gain caused by a large sound is rapidly compensated for in the subsequent recording of small sound, thus the sound recording is not seriously affected by the crashing sound. Because of the rapid retaining of the gain, a small sound after a large sound is recorded at almost the same level as that of the latter, so that the later reproduction will give the motion picture viewer an impression of almost no far and near distance sense. When the sound recording is performed in a noisy environment, the later sound reproduction will give the viewer a noise-accelerated impression. In the motion pictures with simultaneous reproduction of the sound accompaniments, it is of importance to establish proper relationship between the motion picture and the sound accompaniment. Sound reproduction with lack of far and near sense and of the close-up of the sound of principal interest and with the prominance of noise and the lack of correct control of the sound level will give the motion picture viewer a particularly unpleasant impression.